We're having the same issue, and a lot of others seem to be as well. There's a comment from a Comcast employee at the end of this thread noting that they opened up a ticket about the issue: https://forums.xfinity.com/t5/Your-Home-Network/Terabyte-Internet-Data-Usage/td-p/2741624/page/63
Hoping there is a resolution soon. It's clear something is WAY off on their end.
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